The present invention is addressed to the problem of trying to most efficiently fit a round laser beam in a square, triangular or other shaped hole, where the hole is, for example, the aperture in a side-pumped dye laser. Pump laser beams are round and therefore lose efficiency when coupled into apertures having other shapes, such as a square aperture commonly used in a side-pumped dye laser.
Several beam reshaping systems have previously been developed. However, these reshaping systems are complex and not particularly useful for use in laser chemistry and with laser pump beams.
Numerous authors have developed methods of transforming collimated laser beams with Gaussian intensity profiles into other intensity profiles. Circularly symmetric transformations have been developed for creating top-hat intensity profiles from Gaussian intensities. A square beam with uniform intensity can also be created from a Gaussian beam, as well as other separable shapes such as sinc-squared intensity profiles. These transformations make use of the separability of the Gaussian function --that is, exp(-Kr.sup.2)=exp(-Kx.sup.2) exp(-Ky.sup.2). Basically, a rectangular section of the Gaussian profile is remapped into another rectangle with the desired intensity profile. A Gaussian-to-square transform and a good bibliography is described in C. C. Aleksoff, K. K. Ellis and B. D. Neagle, "Holographic conversion of a Gausian beam to a near-field uniform beam." Opt. Eng. 30(5):537-543 (1991).
Bryngdahl proposes a beam reshaping system based on a Fourier optical processor geometry. O. Bryngdahl, "Geometrical transforms in optics," J. Opt. Soc. Amer. 64(8):1092-1099 (1974). This system includes a object plane mask, a Fourier transform lens, the Fourier plane mask, a recollimating lens, and an image plane mask. Both lenses have a focal length L, and the spacing between each successive pair of elements in the system is L. Bryngdahl's scheme locates a beam-aberrating element somewhat similar to ours, in the object plane. This element tilts different parts of the beam so they arrive at different locations on the transform plane. The Fourier mask changes the ray angles so that the second lens will image the ray bundle into the desired shape, for example, a square at the image plane. Finally, a recollimating element is needed in the image plane.
This system can reshape a round laser beam into a square, but is not particularly applicable for laser chemistry or pump beams, and is complex, possessing five elements. Furthermore, lasers used in these applications usually have high-peak powers, so the focus on the Fourier transform plane could damage the mask.
A totally different method of creating a square patch of light with uniform intensity uses a multifaceted mirror. U.S. Pat. No. 4,195,913 (1990); J. M. Geary, "Strip mirror integrator for laser beam uniformity on a target," Opt. Eng 28(8), 859-864 (1989). The faceted mirror is an array of square mirrors, each tilted so that the collection of "beamlets" overlap at an "image plane" a given distance away. The multifaceted mirror approach is a simple and effective method of creating a square beam with uniform intensity. The convergence angles, however, are quite large, so the axial distance over which there is a good "image" is very short. Hence, this method is not appropriate for the applications mentioned above.
Thus, there is need for an improved means of converting round laser beams to other shapes in a way which is simple yet which results in a minimum loss of efficiency.